Muscles Involved in order of importance:

Chest (pectorals)

Front delttoids (shoulders)

Triceps

To perform this exercise lie on a flat bench and take hold of a barbell with a medium width grip,
Then lower the barbell slowly to the middle of your chest breathing in as you lower the bar,
Push the bar straight up to the starting posistion untill your elbows are locked out and remember to exhale as you do this.

Safety: This exercise should be done with a spotter if you think the weight will be a bit too much, I also have a spotter when doing this exercise just to be on the safe side.